(1) Field of the Invention
The present invention generally relates to a pressure sensor for use in an internal combustion engine, and more particularly to a pressure sensor for detecting a pressure of a portion in an internal combustion engine.
(2) Description of the Related Art
A conventional internal combustion engine which is equipped with an electronic control type fuel injector uses several types of pressure sensors. In the internal combustion engine with the electronic control type fuel injector, a basic fuel injection time for which the fuel injector injects fuel is calculated on the basis of an intake manifold pressure and an engine speed in the internal combustion engine which are detected by the sensors, and an air-fuel ratio of air-fuel mixture fed into a combustion chamber of an engine cylinder is controlled so that the air-fuel ratio is maintained to a desired air-fuel ratio. For instance, a vacuum sensor is provided in the engine for detecting the above mentioned intake manifold pressure in an intake passage of the engine, a combustion chamber pressure sensor is used for detecting a pressure in the combustion chamber of the engine cylinder, and a supercharged pressure sensor is provided in the engine with a supercharger for detecting a supercharged pressure in the combustion chamber of the engine cylinder. In some cases, an altitude correction sensor is used for providing an electrical signal in response to an atmospheric pressure at high altitudes, and the atmospheric pressure is corrected by the electrical signal so as to obtain a corrected atmospheric pressure to be used in the engine environment. These sensors of the type mentioned above including the vacuum sensor, the combustion pressure sensor, the supercharged pressure sensor and the altitude correction sensor are hereinafter called a pressure sensor, and in the pressure sensor a piezoelectric element is usually used which generates an electrical signal when it is stressed by a pressure force applied. By receiving this electrical signal from the piezoelectric element, it is possible to detect the pressure applied on a portion in the internal combustion engine at which the pressure sensor is located.
FIG. 1 shows a conventional pressure sensor which is disclosed, for example, in Japanese Patent Application No. 2-130986, and this pressure sensor includes a diaphragm member 1 for converting pressure into mechanical force, the diaphragm member 1 having a pressure receiving part, and a heat insulator 2 for shielding the inside of the pressure sensor from the high-temperature engine environment and for transferring the force from the diaphragm member 1 to a pressure detection part of the pressure sensor by means of a motion of the heat insulator 2 relative to the the pressure detection part of the pressure sensor.
This pressure detection part includes a hemispheric member 3, a piezoelectric member 4, hermetic terminals 5 and a stem 6. The hemispherical member 3 is provided for eliminating residual stresses and load deviations in a direction perpendicular to the axial direction of the diaphragm member 1. An end surface of the heat insulator 2 is joined to an inwardly raised portion of the diaphragm member 1, and the other end surface thereof is brought into contact with one end surface of the hemispheric member 3. The other end surface of the hemispheric member 3 is brought into contact with and fixed to the piezoelectric member 4. The piezoelectric member 4 has a laminated structure including a glass plate 4a, a silicon plate 4b and a glass plate 4c, and is provided for generating an electrical signal in response to the axial force given by the hemispheric member 3. The hermetic terminals 5 are fitted in the stem 6, and the stem 6 is welded to the inside surface of a cylindrical part of the diaphragm member 1. One end surface of the piezoelectric member 4 is mounted on and joined to an end surface of the stem 6. The hermetic terminals 5 serve to pick up the signal generated by the piezoelectric member 4, and the signal is supplied to an external unit via the hermetic terminals 5, so that a pressure in an external environment applied to the diaphragm member 1 is detected.
The stem 6 is welded to the cylindrical part of the diaphragm member 1 through a laser welding process during application of a predetermined constant preload to the pressure receiving part of the diaphragm member 1 from the pressure detection member 4 through the pressure transfer members 2, 3, and those parts are assembled to a condition as shown in FIG. 1 and the applied preload force is retained after the welding is completed. However, it is difficult to keep the axis of the piezoelectric member 4 (or the stem 6) straightly in accordance with the axis of the diaphragm member 1 before and after the welding process. Generally speaking, at the start of the laser welding process, the axis of the piezoelectric member 4 is inclined at an angle "a" with respect to the axis of the diaphragm member 1, as shown in FIG. 2A. The laser welding process is performed so that the stem 6 is permanently joined to the cylindrical part of the diaphragm member 1 by a continuous weld throughout the periphery of the stem 6 at a position 7, as indicated in FIG. 2B. In a case in which the axial direction of the piezoelectric member 4 is inclined at an angle "a" with respect to the axial direction of the diaphragm member 1 at the start of the laser welding, the piezoelectric member's axial direction is, at the ned of the laser welding, inclined at an angle "b" with respect to the diaphragm member's axial direction. Thus, in the course of the laser welding, an inclination angel "a+b" of the piezoelectric member 4 with respect to the axis of the diaphragm member 1 is produced, and owing to the inclination angle, a lateral displacement "d+e" of the axis of the piezoelectric member 4 relative to the axis of the diaphragm member 1 takes place as indicated in FIGS. 2A and 2B.
Although the hemispherical member 3 is provided for preventing a deviating load force from being applied to the piezoelectric element 4 eccentrically form the axial direction of the diaphragm member 1 so as to absorb the above inclination thereof at assembly, one end surface of the hemispherical member 3 is permanentyl joined to teh piezoelectric member 4 and a predetermined constant preload is imparted to the diaphragm member 1 in the axial direction when the stem 6 is welded to the cylindrical part of the diaphragm member 1. Due to the preload applied to the pressure receiving part, the hemispherical member 3 in contact with the heat insulator 2 is not subjected to sliding or motion relative to the heat insulator 2. As a result, the transverse motion of the member 3 is prevented due to the preload, and the inclination of the stem 6 or the lateral displacement (d+e) of the piezoelectric member 4 cannot be absorbed after the laser welding is completed, and the conventional pressure sensor experiences inevitably the above mentioned load deviation, despite the presence of the hemispherical member 3.
FIG. 3A is a diagram for explaining the problems of the conventional pressure sensor and the manufacturing method thereof. In FIG. 3A, it is assumed that the heat insulator 2 is freely moved in relation to the axis of the hemispherical member 3 whose bottom is joined to the piezoelectric member 4. When the heat insulator 2 is shifted from a position indicated by a phantom line in FIG. 3A to a position indicated by a solid line in FIG. 3A in relation to the axis of the piezoelectric member 4 in the course of the above laser welding, a frictional force between the heat insulator 2 and the hemispherical member 3 is produced as a reaction to the lateral displacement described above. Because the frictional coefficient regarding this frictional force is large, the heat insulator 2 does not slide adequately relative to the top of the hemispherical member 3. Therefore, residual stresses on the piezoelectric member 4 are produced as indicated by a solid line in FIG. 3B, and the piezoelectric member 4 experiences load deviations in a direction perpendicular to the axis thereof. A dotted line in FIG. 3B indicates the ideal stress distribution in the piezoelectric member 4 of the conventional apparatus.
In order to prevent the lateral displacement of the axis of the piezoelectric member 4 from being produced, an improved method of producing a pressure sensor has been used in the prior art. FIGS. 4A through 4D show such an improved manufacturing method of the pressure sensor. As shown in FIG. 4A, the diaphragm member 1 is fixed on a work rest and the stem 6 in which the hermetic terminals 5 are fitted is inserted into the diaphragm member 1. The laser welding is performed while a predetermined constant preload is applied to the diaphragm member 1 from the stem 6 through the hemispherical member 3. This laser welding is performed simultaneously at three positions 8a, 8b, 8c on the outer periphery of the stem 6, as in a sectional view shown in FIG. 4B. The diaphragm member 1 to which the stem is welded at the positions 8a, 8b, 8c is then rotated by a given rotation angle about the axis of the stem 6 as shown in FIG. 4C, and the laser welding is again performed to weld the stem 6 simultaneously at three other positions 8d, 8e, 8f on the outer periphery of the stem 6 as in a sectional view shown in FIG. 4D. The stem 6 is welded to the cylindrical part of the diaphragm member 1 by performing repeatedly the three-point simultaneous welding process. According to this improved manufacturing method, the above mentioned inclination angle of the axis of the piezoelectric member 4 as a result of the welding process can be reduced to a smaller level than that when the conventional manufacturing method is used.
However, in a case in which the axis of the stem 6 is initially inclined at an angle with the axis of the diaphragm member 1 before the preload is imparted to the pressure receiving part, or in a case in which the welded portions shrink at different shrinkage rates after the laser welding is performed, the axis of the piezoelectric member 4 actually slant at an angle with the axis of the diaphragm member 1 if the improved manufacturing method is used. Thus, there is a problem in that the conventional pressure sensor may provide an inaccurate pressure-responsive signal due to the inclination of the axis of the piezoelectric member 4.